Conventionally, an electrophotographic photosensitive member containing amorphous silicon (hereinafter also referred to as “a-Si”) is manufactured by forming deposited films such as a photoconductive layer on a cylindrical base member. Deposited film forming methods, which have been widely used, include a method of decomposing a raw material gas for deposited film formation using an RF band high-frequency glow discharge and attaching the decomposed products to the cylindrical base member, called an RF plasma CVD method.
Demand for improved image quality of electrophotographic devices has increased in recent years, and an improved uniformity of deposited films of an electrophotographic photosensitive member (thickness and quality uniformity of deposited films) and improved characteristics of the deposited films have been increasingly demanded accordingly.
In the conventional RF plasma CVD method, because of the frequency being high, standing waves may be produced in accordance with the wavelength to form a low-electric-field portion in a plasma, or propagation variation due to the impedance of the plasma CVD device to be used may cause a non-uniformity in plasma, which makes the improvement of the uniformity of deposited films a challenge. In addition, since the electric field to be used is an alternating electric field, charged particles (ions and electrons) in a plasma reciprocate due to the electric field, and may, during reciprocation, undergo a secondary reaction with other charged particles, neutral active species, or raw material gases to become powdery matter. The powdery matter is taken into deposited films, which makes the improvement of the characteristics of the deposited films a challenge.
Among the above challenges, in terms of the improvement of the uniformity of deposited films, the use of low-frequency glow discharge, on which the influence of standing waves or the impedance of the plasma CVD device is reduced, has been studied. In terms of the improvement of the characteristics of deposited films, in order to prevent secondary reactions in a plasma, adjusting all the voltages to have either positive or negative polarity, that is, applying only voltages of either polarity and causing the voltages to discharge, has been studied. Hereinafter, applying only voltages of either positive or negative polarity and causing the voltages to discharge is referred to as “unipolar discharge”, and alternately applying voltages of both positive and negative polarities and causing the voltages to discharge is referred to as “bipolar discharge”.
PTL 1 discloses a technology in which rectangular wave voltages of only either positive or negative polarity whose frequency is not more than 300 kHz is used. According to PTL 1, a low frequency of 300 kHz or less improves the uniformity of deposited films. Furthermore, bipolar discharge as disclosed in PTL 2 may cause the secondary reaction described above even if the frequency is not more than 300 kHz, whereas unipolar discharge would allow charged particles (ions and electrons) to move only in one direction and therefore prevent the secondary reaction described above.